Mobile systems and displays demand efficient and longer battery usage. Additionally, display quality is the most important performance feature that cannot be compromised even during heavy load current fluctuation, large input voltage transition and switching noise from the DC (direct current)-DC converters.
The active matrix OLED (AMOLED) display becomes very popular for mobile display applications owing to its advantages such as high display quality, low power consumption and low material cost. The AMOLED panel usually requires both positive and negative power supplies with different regulated voltages. The display quality is also depending on the voltage ripple of the two power supplies. Every panel has different output current and voltage levels requirements depending on such as panel size, pixel numbers, display quality and the like.
FIG. 1 shows a conventional single inductor AMOLED power supply, which is a two-stage SIBO converter. As shown in FIG. 1, the conventional two-stage SIBO converter 100 includes a synchronous buck-boost circuit 120, a charge pump 140, an inductor L11 and capacitors C11-C15. The capacitors C11-C13 are decoupling capacitors. The capacitors C14-C15 are fly capacitors. The conventional two-stage SIBO converter 100 generates a positive output Vop for driving the load 160 by the positive current lop, and a negative output Von for driving the load 180 by the negative current Ion. The input provides the input voltage Vin and the input current lin.
The synchronous buck-boost circuit 120 may operate at buck, buck-boost or boost modes, depending on the input voltage Vin and the output voltage Vop conditions. The input voltage Vin, which is usually provided by a Li-Ion battery, ranges from 3.0V to 4.5V. The output voltage Vop depends on AMOLED panel size, brightness and driver IC, and typical values of the output voltage Vop include 4.6V, 3.3V, 2.8V or 2.5V, etc.
The charge pump 140 is configured to generate the negative output Von from the positive output Vop. The charge pump 140 has many output steps, for example but not limited to −1× and −1.5×. By using the fly capacitor C14, the charge pump 140 may implement the step −1×, that is, Von=Vop*(−1). By using both the fly capacitors C14 and C15, the charge pump 140 may implement the step −1.5×, that is, Von=Vop*(−1.5). The negative output Von may be programmable from around −1× to −1.5× of the positive output Vop for high brightness situation in an AOMLED display.
From FIG. 1, the generation of the positive output Vop and the negative output Von are independently controlled.
FIG. 2 shows the conversion efficiency of the two-stage SIBO converter 100. The conversion efficiency Eff is defined as:
  Eff  =                                                    Iop            *            Vop                                    +                                        Ion            *            Von                                                                  Iin          *          Vin                              *    100    ⁢    %  
As shown in FIG. 2, the efficiency of the conventional two-stage SIBO converter 100 is at peak when Von=Vop*(−1)=2.8*(−1)=−2.8(V) or Von=Vop*(−1.5)=2.8*(−1.5)=−4.2(V) if Vop=2.8(V). However, the efficiency of the conventional two-stage SIBO converter 100 is not good when Von is neither −2.8(V) nor −4.2(V). Therefore, the efficiency of the conventional two-stage SIBO converter needs to be improved.